MEMs technology is being used increasingly in electronic devices. Switches and tunable capacitors are examples of circuit components that can be made using MEMs technology.
MEMs switches can be used in a wide variety of applications, where high speed, typically low current, switching is required within a circuit. A MEMs switch has the advantage of a large capacitance switching range.
MEMs capacitors can also be used in a variety of circuits, such as tunable filters, tunable phase shifters and tunable antennas. One application of growing interest is in RF and microwave communications systems, for example for use in low cost reconfigurable/tunable antennas.
FIG. 1 shows a MEMS capacitor with an electrically tunable dielectric and MEMS controlled dielectric spacing. The dielectric spacing is controlled in the manner of a switch, but the analogue electrical control of the dielectric properties can enable continuous tunability of the capacitor.
A tunable dielectric, ferroelectric or piezoelectric material can be used, such as Ba1-xSrxTiO3 or PZT as a dielectric layer 14. By combining a MEMS capacitor with a tunable dielectric, the advantages of the large capacitance switching range of RF MEMS switches are added to the advantages of the continuous tuning capability of tunable dielectrics. Moreover, use is made of the beneficial high dielectric constant of ferroelectrics, which can be 10-200 times higher than that of conventional dielectrics like Silicon Nitride. This dramatically reduces device size and increases continuous tuning range.
The device comprises opposite capacitor plates 10 (e1)) and 12 (e2). The gap g is controlled by the MEMS switch represented by the spring k, based on the voltage applied to the plate 12. A dc voltage Vdc_switch is used to provide this MEMS switching function, from a dc voltage source 18. An rf ac voltage source 16 represents the rf signal that is flowing through the MEMS device during operation. The tunable dielectric has a tunable dielectric value ∈d, whereas the remaining dielectric spacing is air or vacuum, with dielectric value ∈0. The tunable dielectric is controlled by the voltage Vdc_tune, so that the single voltage applied to the electrode 12 controls the MEMS switching and dielectric tuning. The capacitor C and resistor R are optional decoupling components.
Of course, if a non-tunable dielectric is used, the MEMs device can be used simply as a capacitive switch (low frequency or rf frequency). Alternatively, if no dielectric is provided, the device can be used as a galvanic switch.
Switches/tunable capacitors of the type shown in FIG. 1 are usually operated in a gas atmosphere at a pressure close to 1 bar. A drawback of the presence of this gas is that it exerts a damping force on the moving electrode of the switch, which reduces its speed. Because the gas needs to be ‘squeezed’ away between the electrodes, this damping force is sometimes called the ‘squeeze film damping’ force, and is approximately given by:
                              F          d                =                              -            b                    ⁢                      v                          z              3                                                          [        1        ]            
In equation [1], v is the speed of the electrode, b is a constant that depends on the geometry of the switch, and z is the separation between the plates. Clearly, the damping force increases tremendously when the plates approach each other and z becomes small. This strongly limits the switching time both for opening and for closing of the switch.
An obvious and well-known solution to this problem is to operate the switch in a vacuum or low pressure. At the limit, the gas damping force becomes close to zero and the switch closes and opens very quickly.
However this solution does have another drawback: when the switch opens, the spring-mass system is strongly underdamped and the electrode will oscillate for a long time before finally coming to a relaxed position. This increases the effective opening time (usually the switch can only be used after stabilizing, particularly in the case of capacitive MEMS switches).
FIG. 2 shows the opening of a capacitive MEMs switch at atmospheric pressure (1000 mbar). The opening speed is low, especially when the electrode gap z is very small.
FIG. 3 shows the opening of the same capacitive MEMs switch at a low pressure (1.4 mbar). Because the gas damping is very small, it takes many oscillations for the switch to relax to a stable position.
According to the invention, there is provided a MEMS device comprising:
first and second opposing electrodes, wherein the second electrode is electrically movable to vary the electrode spacing between facing first sides of the first and second electrodes, wherein the second electrode is at a spaced position from the first electrode in a relaxed state in which no voltages are applied to the electrodes,
wherein a first gas chamber is provided between the electrodes, at a first pressure, and a second gas chamber is provided on the second, opposite, side of the second electrode at a second pressure which is higher than the first pressure.
This arrangement enables movement of the second electrode towards the first electrode without needing to displace significant volumes of fluid, as the chamber between the electrodes is at a relatively low pressure. The movement of the second electrode towards the relaxed position is damped by the second gas chamber so as to prevent unwanted oscillations. The invention thus provides the ability for high speed switching, whilst also reducing settling times.
The first gas chamber is preferably at a pressure below 500 mbar, and the second gas chamber is at atmospheric pressure or higher than atmospheric pressure. For example, the first gas chamber can be at a pressure below 100 mbar, or even below 50 mbar, namely at a partial vacuum pressure. The second gas chamber can for example be at atmospheric pressure. This simplifies the manufacture as the chamber does not need to be pressurised or depressurized as part of the manufacturing process.
The first gas chamber can have a height of between 50 nm and 50 μm and the second gas chamber can have a height of between 1 μm and 200 μm. The electrostatic control is across the first gas chamber, which has a relatively small height, whereas the second gas chamber is used for mechanical damping only, and therefore can have greater height.
One or more stops can be provided in the second gas chamber, for limiting movement of the second electrode away from the first electrode. These can assist further in damping oscillations as the second electrode moves to its relaxed state.
The second electrode can be clamped between elastic members around a portion of the second electrode which defines the outer periphery of the first and second gas chambers. Again, this can be used to provide damping of the movement of the second electrode.
The second electrode can comprise a metal electrode layer, a piezoelectric layer and a circuit for dissipating electrical energy generated by the piezoelectric layer. This arrangement uses the conversion of mechanical to electrical energy as a further possible way to provide damping of the movement of the second electrode. A circuit can then be provided for controlling the actuation of the piezoelectric layer to provide active damping.
The device of the invention can for example comprises a MEMs switch (either a capacitive switch having a dielectric layer between the first and second electrodes, or a galvanic switch in which the second electrode is movable to make and break contact with the first electrode).
The invention also provides a method of controlling the movement of an electrically movable electrode of a MEMS device towards and away from a fixed electrode, wherein the movable electrode is at a spaced position from the fixed electrode in a relaxed state in which no voltages are applied to the electrodes, the method comprising:
providing a first gas chamber provided between the electrodes with a first pressure;
providing a second gas chamber provided on an opposite side of the movable electrode facing away from the fixed electrode with a second pressure which is higher than the first pressure.